The demand for artificial blood vessels, and in particular, artificial arteries is great. This is due to the fact that over 600,000 surgical procedures involving large and small blood vessels are conducted annually.
Arteries are complex structures, as the cellular and extracellular components of the artery wall are not uniformly distributed. Rather, these cellular and extracellular components are organized into discrete layers. These discrete layers have a trilaminant structure of an intima, a media and an adventitia.
The mechanical properties of the artery wall are largely due to components of the vessel media layer. Elastin is the most extensible component of the vessel media, whereas collagen is relatively stiff. The presence of smooth muscle cells is important for two reasons. First, the smooth muscle cells synthesize the elastin and collagen present in the vessel media and second, they are capable of contraction and relaxation in response to various external stimuli including vasoactive substances in the blood and sympathetic and parasympathetic nerve impulses. These smooth muscle cells are thus an important source of structural protein that contribute directly to the strength and elasticity of the vessel wall and represent a component of variable elasticity participating in the regulation of vessel tone.
The elastin and collagen components confer a non-linear stress-strain relationship, in which the elastic modulus of the vessel wall increases (the vessel becomes stiffer) as the vessel is distended. The importance of organization to mechanical function in the arterial wall is clearly seen when arteries are compared to veins of similar size. In arteries, the circumferentially or (helically) oriented smooth muscle cells and associated elastin and collagen of the media, provide the mechanical strength necessary to withstand the higher pressures that exist in the arterial circulation. In contrast, the media layer of a vein of similar size is reduced in thickness, contains fewer smooth muscle cells, less elastin, and lastly little circumferential orientation of smooth muscle collagen and elastin seen as in the arteries.
Several attempts have been made at creating a completely bioartificial artery. Weinberg and Bell, "A Blood Vessel Model Construction from Collagen and Cultured Vascular Cells" in Science, Vol. 231 (1986), pp. 397-400, reported production of an artery involving smooth muscle cells and fibroblasts in layers of reconstituted Type I collagen that approximate the laminate structure, but not the circumferential orientation of natural arteries. These vessel analogs lacked the mechanical strength necessary to withstand the stress associated with pulsatile blood flow in vivo. To increase the strength, proponents of the Weinberg and Bell approach were forced to reinforce these arteries with synthetic polymer sheaths, such as Dacron.RTM.. These bioartificial arteries exhibit a severe drawback in that biocompatability problems can ultimately lead to graft failure as reported by Langer and Vacanti, "Tissue Engineering" in Science, Vol. 260 (1993), pp. 920-926.
The orientation of fibrils and cells in collagen has been studied. For example, Torbet and Ronziere, "Magnetic Alignment of Collagen During Self-Assembly" in Biochem. J., Vol. 219 (1984), pp. 1057-1059, reported collagen fibril orientation in the presence of a magnetic field. Additionally, Guido and Tranquillo, "A Methodology for the Systematic and Quantitative Study of Cell Contact Guidance in Oriented Collagen Gels" in Journal of Cell Science, Vol. 105 (1993), pp. 317-331, reported that cells orient in the same direction as the oriented collagen fibrils, under the phenomenon known as "cell contact guidance".
Despite the study of fibril and cell orientation, there remains the need to provide a tissue-equivalent tube that has mechanical properties that enable it to better withstand distention, typically associated with pulsatile blood flow in arteries, without reinforcing synthetic materials.
Additionally, regeneration of severed peripheral nerves commonly results in permanent damage and disability because mature neurons do not normally replicate. Once severed, the severed ends of the neurons usually grow in a misdirected manner, such that they do not reestablish the original connection. Numerous attempts have been made to reestablish the connections, including the placement of a tube filled with a matrix for bridging the nerve endings. For Example, Rosen, "Artificial Nerve Graft Using Collagen as an Extracelluar Matrix for Nerve Repair Compared with Suture Autograft in a Rat Model", in Annals of Plastic Surgery, 25:375-387 (1990) reported slightly inferior nerve regeneration using tubes filled with collagen gel as compared to nerve autografts, whereas Valentini, et al., "Collagen and Laminin Containing Gels Impede Peripheral Nerve Regeneration through Semipermeable Nerve Guidance Channels", in Experimental Neurology, 98:350-356 (1987) reported greatly inferior nerve regeneration as compared to saline-filled tubes.